The present invention relates to a device for contactless, continuous casting of metals in the vertically downwards direction in an electromagnetic alternating field having an inductor, a coolant chamber featuring a device for directing a coolant onto the surface of the cast metal strand or ingot, and a electrically conductive non-ferromagnetic scrren that tapers downwards and is situated above the plane defined by the lowest lying edge of the inductor.
Also within the scope of the present inveniton is a process for contactless, vertical downwards continuous casting of metals in such a device, and an application of this process.
With such electromagnetic continuous casting units the molten metal is poured on to a dummy base situated within an inductor loop. The dummy base is lowered at a give rate. A high frequency alternating current in the inductor creates an electromagnetic field which restrains the molten metal horizontally within the inductor in a shape which is determined essentially by innter contours of the inductor loop. By jetting the surface of the downwards moving strand with a coolant, for example water, rapid solidification of the surface layer takes place. In order to adjust the magnetic field forces to accommodate the metallostatic pressure in the molten part of the strand, a scrren for example of stainless steel and also in the form of a loop is situated within the inductor loop.
A significatn advantage of the electromagnetic continuous casting unit over the conventional type is the much more uniform surface of the cast strand or ingot which is free of cold shuts, bleeding and surface segregation as a result of which scalping is unnecessary in most cases.
Further design aspects of such electromagnetic molds are aimed at correcting any inconsistencies in flatness and solidifcation conditions. For example the patent DE-C-2 No. 848 808 describes a mold which prevents concavity in broad rolling ingots by special shaping of the inductor. The patent EP-B-0 No. 015 870 proposes a fine regulation of the angle and region of impact of the coolant by controlled deflection of the coolant stream, this in order to be able to adjust the solidification conditions optimally to suit the various alloys and casting speeds. Proposed in patent EP-B- No. 062 606, as a means of avoiding convex doming of the bottom end of the ingot due to nonstationary cooling conditions during the start of the drop, is a deflection surface with recesses that is moveable parallel to the axis of the ingot and is inserted in the path of the coolant at least during the start-up phase. The patent EP-B- No. 0 082 810 describes a further method for reducing the doming of the ingot bottom caused by extreme cooling of the ingot. In that case, at least at the start of casting an addition is made to the coolant viz., a substance which on contacting the hot ingot surface decomposes to form a gas and thus forms an insulating film there that reduces the rate of that extraction.
Described in patent EP-B- No. 0 109 357 is the design of an electromagnetic continuous casting mold which can be adjusted for different ingot cross-sections without reducing the dimensional accuracy of the ingot contour.
The electromagnetic field produced by the inductor creates in the molten head of the ingot being cast circulation of the melt which can for example cause the oxide skin to rupture. In such sensitive cases this disturbs the solidification conditions and lowers the quality of the melt in the region where the surface is solidifying; this then appears e.g. as an agglommeration of oxide inclusions, in longitudinal folds and in surface flaws of a kind which do not appear until later processing viz., as surface fines, looper lines and the like. Such ingots usually have to be scalped so that the advantages of electromagnetic casting cannot be fully exploited.
In view of the above the object of the present invention is to develop a device of the above mentioned kind by means of which the surface quality of cast ingot or strand and the products manufactured therefrom are improved, and in particular the circulation of the melt in the molten head of the ingot or strand can be reduced.
It is a further object of the invention to develop a process which improves the surface quality obtained by contactless, vertical downwards continuous casting of metals.